Hitherto, in a seatbelt-wearing condition in which a seatbelt is worn by a motor vehicle occupant seated in a motor vehicle seat, various technologies for blocking occurrence of a phenomenon, i.e., a so-called submarine phenomenon, in which the motor vehicle occupant has a behavior to slip through a downside of a seatbelt along a seat surface, along with a frontward movement of a waist portion of the motor vehicle occupant when in occurrence of an accident, are proposed. For example, in Japanese Unexamined Patent Application Publication No. 5-229378, a motor vehicle seat having a construction in which an expanded airbag compresses a seat cushion from below so as to block the movement of the motor vehicle occupant toward a front side of the motor vehicle at the time of occurrence of the accident is disclosed. However, in designing such a kind of motor vehicle seat, it is supposed that water, moisture, or the like flows into the airbag disposed at a lower portion because the airbag is installed below the seat cushion. Therefore, there is a demand for a technology to prevent the airbag from absorbing moisture.